Airborne Differential Absorption Lidar
Development of a novel Airborne Differential Absorption Lidar System for the UK FAAM aircraft.
An advanced Differential Absorption Lidar (DIAL) instrument is being developed in the UK for operation on board the NERC Facility for Airborne Atmospheric Measurements (FAAM) BAE 146-301 Large Atmospheric Research Aircraft G-LUXE as part of a Joint Infrastructure Fund / NERC award to the University of Cambridge and UMIST.
The DIAL instrument will make range-resolved concentration measurements of water vapour and ozone, in addition to aerosol backscatter and polarisation measurements. The system will be able to operate in nadir or zenith pointing configurations, which, from a typical flight altitude, will provide high-resolution profiles throughout the troposphere and into the lower stratosphere.
The instrument will provide near real-time profiles, complementing and extending the data sets currently available from sondes, ground based radar, and the other FAAM instrumentation. The performance objectives for the DIAL instrument are summarised below.
| Parameter | Ozone concentration profile |
Water vapour concentration profile |
Aerosol backscatter profile |
| Minimum vertical resolution | = 300 m | = 300 m | ~ 30 m |
| Minimum time resolution | = 1 minute | = 1 minute | = 1 minute |
| Maximum range | = 5 km | = 5 km | 10 km |
| Accuracy | 10% or 5 ppb | 10% | 10% |
| Notes | Strong and weak lines |
Polarisation ratio also measured |
Performance specification of the new DIAL instrument
The National Physical Laboratory (NPL) is carrying out the overall design, development and integration of the system for the University of Cambridge. NPL have developed and operated a number of mobile ground based lidar systems. The laser source is an injection seeded Ti-Sapphire multi-wavelength system being supplied by a US specialist laser supplier (Q-Peak Inc) to meet the specific requirements of this project. The laser is a dual Nd-YAG pumped Ti-Sapphire. Wavelength seeding is provided by a series of widely tuneable ECDL lasers. Wavelength diagnostics will be provided by online wavemeters and pulse-by-pulse measurements through gas cells.
The system will operate at 25 Hz, switching sequentially between five wavelengths. Three of these will be used for near-infrared measurements of water vapour, the other two wavelengths will be frequency tripled to provide ultraviolet pulses for ozone measurements. Aerosol measurements will be made using the infrared wavelengths. Each wavelength will be independently tuneable to enable the operation of the system to be tailored to different scientific measurement requirements.
| Ultraviolet Wavelengths (spectral line width < 50 pm) |
Infrared Wavelengths (spectral line width < 1 pm) |
| 295 ± 2 nm | 942.3 nm |
| 308 ± 2 nm | 943.1 nm |
| 943.7 nm |
Typical wavelengths for DIAL operation
The beam will be expanded and intentionally made sufficiently divergent to provide an eye-safe limit compatible with nadir operation at the standard cruise altitude. One of the principle issues with an airborne lidar installation is the necessity for windows in the aircraft hull through which the beam must pass. A number of features have been incorporated into these to reduce the problems associated with placing a window in a lidar beam while meeting the requirements for airworthiness certification.
A 40 cm diameter Dall-Kirkham telescope will be used to collect the backscattered radiation. The detection system will operate in both photon-counting and analogue modes, to provide the dynamic range required to achieve a measurement range of 10 km in both nadir and zenith directions. In addition the infrared channels will be split to detect polarisation. The system will therefore digitise 16 channels of data, at up to 20 MHz resolution, for each DIAL measurement (five wavelengths including polarisation).
A further design consideration has been the issue of developing a system that meets the stringent requirements for operation onboard a civil aircraft in the UK. This has led to a number of constraints and design considerations not normally required for a lidar system. The laser and detection systems will be mounted on an anti-vibration mounted suspended framework. Ancillary electronics and control systems will be mounted on two additional rack units forward and aft of the main system.
A comprehensive simulation model has been developed and implemented by the University of Cambridge. This models the atmospheric backscatter from the Lidar pulses and has been used to predict the performance of the system in key atmospheric research scenarios.
For further information contact Rod Robinson at NPL or Rod Jones at the University of Cambridge. For further information on the FAAM aircraft and schedule visit www.faam.ac.uk